The truth about cholesterol
The Truth about cholesterol – Cholesterol has been associated with heart disease to such an extent that many of us instinctively watch our consumption of foods known to contain it and view such foods as “unhealthy”. The truth is however that cholesterol is essential to health.
Cholesterol is not a fat, it is a sterol (a contraction of alcohol and steroid which is safe legal alternatives) alcohol made in the liver. It is not water-soluble and cannot actually be transported freely in the blood; instead it is carried in special proteins known as lipoproteins and distributed throughout the body. It has a number of vital functions:
- Cholesterol is a precursor to the sex hormones testosterone, estrogen, androgen and progesterone; it is also a precursor to corticosteroids including cortisol and DHEA, which regulate the stress response.
- Cholesterol is a precursor to vitamin D, an essential nutrient required for normal calcium utilisation that has also been linked to a reduced risk of multiple sclerosis and chronic disease
- Cholesterol is present in the fatty membrane surrounding every cell in the body; in this structural role cholesterol maintains the membrane fluidity and selective permeability required for normal cell signaling and nervous system function.
- Serotonin receptors in the brain require cholesterol in order to function properly. Serotonin is a neurotransmitter that elevates mood and reduces pain perception; low levels of serotonin are linked to depression, aggression and suicide.
- In the liver, cholesterol is converted to bile, which is then stored in the gallbladder. Bile salts emulsify fats in the digestive tract and aid in the intestinal absorption of fat molecules as well as the fat soluble vitamins, Vitamin A, Vitamin D, Vitamin E and Vitamin K.
The truth about Cholesterol and heart disease
There are many types of lipoproteins within blood which transport cholesterol to different target tissues; they are called chylomicrons, very low density lipoprotein (VLDL), intermediate density lipoprotein (IDL), low density lipoprotein (LDL), and high density lipoprotein (HDL). The lipoproteins bind to specific receptors on cell membranes, ensuring that cholesterol is transported to target tissues.
LDL is the major carrier of cholesterol and it believed to be responsible for the link between cholesterol and heart disease. When a cell has an abundance of cholesterol the synthesis of LDL receptors is inhibited to prevent further take up of cholesterol by the cell. In some circumstances LDL molecules appear in the blood without receptors on the peripheral tissues to take it up; these LDL molecules become oxidized and can become trapped in the walls of blood vessels and contribute to the formation of plaques on artery walls that cause heart disease.
HDL particles are thought to transport cholesterol back to the liver for excretion or to other tissues that use cholesterol to synthesize hormones in a process known as reverse cholesterol transport (RCT). A high ratio of HDL to LDL in the blood is associated with a reduced risk of heart disease.
It will be noted from this description that cholesterol does not directly cause heart disease; how cholesterol accumulates in arterial walls is not fully understood. Many scientists question the cholesterol/heart disease hypothesis; for example the Masai people in Africahave an extremely high intake of animal fat yet coronary heart disease is almost unknown among them.
Cholesterol and diet
The evidence is abundant that consumption of total fat is not linked to heart disease. The total amount of fats in our diet today is virtually the same as it was at the beginning of this century. What has changed is the types of fats eaten. At the turn of the century we ate mainly animal fats that are largely saturated and monounsaturated. Now we are tending to eat more polyunsaturated fats and trans-fats, with a much increased consumption of refined carbohydrates and sugar.
Dietary cholesterol is known not to influence blood cholesterol levels. High levels of saturated fat (found in meat and dairy) in the diet are thought to raise cholesterol levels, but even this is not conclusive. The massive US MR-FIT trial involved over 350,000 men at risk of heart disease. One group of these men reduced their intake of saturated fat by 28%, cholesterol by 42% and calories by 21%. The authors concluded: “the overall results do not show a beneficial effect on CVD or total mortality from this multifactorial intervention”. The director of another massive study, the Framingham study, concluded: “the more saturated fat one ate, the more cholesterol one ate, the more calories one ate, the lower the person’s serum cholesterol”, indeed these individuals “weighed the least and were the most physically active”. One particular saturated fat, the dominant fat in chocolate, known as stearic acid, is known not to raise cholesterol; indeed there is currently no evidence that a low saturated fat diet alone prevents heart disease.
Based on current evidence, replacement of total, unsaturated, and saturated fats with refined, high-glycemic index carbohydrates is unlikely to reduce heart disease risk and may increase risk in persons predisposed to insulin resistance. In contrast, a diet rich in whole grains and other minimally processed carbohydrates that includes sensible amounts of fats (approximately 30%-40% of total energy), particularly omega-3 fats from seafood, while being low in refined carbohydrates and largely free of foods containing trans fatty acids (hydrogenated vegetable oil), will likely reduce the risk of heart disease. Importantly consuming the correct amount of calories according to body need and not more will reduce the synthesis and storage of fat and consequent risk of heart disease.
The conclusion is of course: all things in moderation. A balanced diet based on whole-grain carbohydrates such as porridge oats, high quality protein including eggs, free range meat and oily fish, fruits, vegetables, pulses, antioxidant rich foods such as green tea and dark chocolate, sensible amounts of dairy, nuts, avocadoes, butter and olive oil, combined with regular physical activity will help ensure optimal health throughout life.
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